This invention relates to the control of a diesel engine, and more specifically, relates to the use of skip firing of the engine to reduce smoke emissions.
The technique of eliminating the firing of selected cylinders in an internal combustion or diesel engine is referred to as xe2x80x9cskip firingxe2x80x9d. Removing the fuel supply (and/or spark ignition in a spark ignition engine) from these cylinders prevents them from firing. This technique has been used in the prior art to improve certain aspects of engine performance. When skip firing is initiated, the fuel quantity removed from the skipped cylinders must be added to the firing cylinders so that the performance parameters of the engine are not significantly changed.
Large self-propelled traction vehicles, such as locomotives, typically use a diesel engine to drive a three-phase alternator (having a rotor mechanically coupled to the output shaft of the engine) for supplying electric current to one or more traction motors having rotors drivingly coupled (through speed reducing gearing) to axle-wheel sets of the vehicle. When excitation current is supplied to the field winding of the alternator rotor, alternating voltages are generated in the three-phase stator windings. The three-phase voltages are applied to input terminals of at least one three-phase, bi-directional power rectifier. If the locomotive has DC traction motors, then the rectified voltage is supplied to the parallel connected armature windings of the traction motors via a link. If the locomotive is equipped with AC rather than DC motors, then an inverter is interposed between the power rectifier and the traction motors to supply variable frequency power to the AC motors.
For the purpose of varying and regulating the speed of the diesel engine, it is common practice to equip the engine with a speed regulating governor that adjusts the quantity of pressurized diesel fuel injected into each engine cylinder. In this way, the actual speed (RPM) of the crank shaft is controlled and corresponds to a desired engine speed which is associated with the desired engine horsepower. In a typical electronic fuel injection system, the output signal from the speed regulating governor drives individual fuel injection pumps for each cylinder, thus allowing the controller to individually control the fuel value (i.e., amount of diesel fuel) injected into each cylinder. The desired engine speed and load is set by manually operating a lever or handle on the throttle that can be selectively moved through eight motoring steps or notch positions by the locomotive operator. In addition to the eight power notch positions, the handle has an idle position and several dynamic braking positions. In these dynamic braking positions the traction motors are operated as generators to produce current that is dissipated by passing through resistance banks. These resistance banks are cooled by fans operating at a speed determined by the engine speed.
When not in use, the locomotive is typically parked with its engine running, its throttle in the idle position, and its main alternator developing no power (i.e., because there is zero traction load). However, the typical engine idle speed is set high enough to power all engine-driven auxiliary equipment operative in the idle mode. Further, to conserve fuel, it is also a known practice to reduce engine speed below the regular idle setting (i.e., to a preselected low idle speed) such as 335 RPM (so long as the desired engine performance parameters remain within appropriate tolerance limits). Although the low idle speed conserves fuel and reduces overall stress on the engine, it can also cause excessive smoke generation.
Excessive engine smoke is generally caused by two different diesel engine operating conditions. If the fuel to air ratio is high (i.e., too much fuel relative to the amount of air in the cylinder), excessive smoke is generated because the quantity of air is insufficient to provide complete burning of the fuel. This is especially prevalent at high loads where too much fuel is injected for the quantity of air. This condition also occurs during speed increases until the air quantity has increased to accommodate the higher injected fuel value. Excessive smoke is also caused by poor fuel atomization. The latter cause is prevalent at engine idle and low notch positions.
Fuel injection pressure is critical to smoke formation because fuel injected at higher pressures breaks up or atomizes better as it enters the combustion chamber. Better atomization allows air to mix with the fuel creating a higher localized air-fuel ratio within the cylinder, fostering complete burning and low smoke production. On a specific fuel injection system with a defined pump, nozzle, cam profile, and operating speed, the injection pressure is governed by the injection duration. As the injection duration increases within the cam profile, the injection pressure goes up. Conversely, as the injection duration decreases, the injection pressure decreases. The latter is especially prevalent at engine idle or other unloaded conditions such as dynamic braking. In fact, there are two aspects to idle operation that promote excessive smoke due to low fuel pressure. First, idle conditions are unloaded, so injection durations are very short because the fuel value is very small. Also, idle engine speeds are generally low, which create lower cam velocities. Both conditions significantly reduce the injection pressure, causing an increase in smoke production.
Engine components (cams, bearings, pumps, injectors, etc.) are designed to a maximum peak injection pressure limit. This prevents making mechanical changes to the fuel system to increase idle injection pressure and thereby reduce smoke production, such as, a faster cam profile or smaller injector spray nozzle holes. Such design changes would raise peak injection pressure at all operating points, and at full load (notch 8), the peak injection pressure may exceed design limits.
The recent enactment of environmental statutes and the promulgation of related regulations by the Environmental Protection Agency require reduction in smoke emissions from diesel locomotives. Locomotive manufacturers are therefore directing attention to reducing smoke emissions to comply with these regulations.
It is known that advancing engine timing reduces smoke output by extending the burn time of the fuel as the fuel/air mixture is in a highly compressed state. It is also known that advancing timing increase the formation of NOx, which are also limited by environmental regulations.
The system and method of the present invention overcomes the limitations and disadvantages of the prior art with respect to the production of visible smoke during low power operation of diesel engines, such as when a locomotive is operating in a dynamic braking mode or in an idle state, by skip firing the engine according to the teachings of the present invention when certain operating conditions are satisfied.
Typically dynamic braking throttle positions command a high speed (to produce a high fan speed to dissipate the heat generated by the traction motor generated current as the traction motors operate as generators) but low power requirements because the engine load is low. The higher engine speed produces a higher fuel injector cam speed, but the fuel injection duration is lower because less power is required. The injection duration is so short that just as the fuel pressure is building the injection is terminated. The result is poor fuel atomization and excess smoke. By skip firing the diesel engine, the smoke emissions are reduced. But it is critical to determine the conditions under which skip firing can be implemented without adversely impacting the power required by the various locomotive systems.
Even when the locomotive is parked at idle, certain auxiliary systems load the diesel engine and thus it is required that the engine operate at some minimal power output level during a skip firing period. The present invention also provides an apparatus and method for avoiding engine speed transients caused by the initiation and termination of engine skip firing and for changing the engine timing during the skip firing period.